1. Field of the Invention
The present invention relates to a method and apparatus for transmitting information to an electromagnetic receiving and control device connected to underground pipework having a substantial length. Specifically, the present invention relates to fluid transport equipment, in particular underground fluid transport equipment, in very deep vertical, diagonal or horizontal pipework, such as the pipework used, for example, in various drilling and fluid transport equipment of oil or gas drilling and production operations.
2. State of the Prior Art
With regard to drilling, and in particular regard to oil wells, it is important to be able to transmit from the bottom of the drill well to the surface the information gathered by measurement sensors. The sensors supply useful information for the subsequent working of the site, for example data regarding pressure, the nature of the solids and fluids encountered, the temperature, etc. It is also important to be able to transmit orders from the surface controlling various equipment and devices such as valves, protective covers, etc., which are found at the bottom of the drilling or production operation.
Traditionally, the transmission of such data requires that an electric cable be lowered into the well, the lower part of the cable being connected to measurement sensors, and the upper part of the cable being handled by a special winch and electrically fed, and connected to processing apparatus for processing the signals indicating the measurements of the sensors. However, a cable incorporating such sensors, or being connected to control sensors or devices, located at a substantial depth cannot be lowered in every situation. In particular, the cable may not be able to be lowered when a drill well is being operated, or when valves or separation devices cannot be crossed by a cable, whether or not the cable is fitted with sensors.
Another method of transmitting data has been termed the "downhole measurement while drilling" technique (DHMWD, or simply MWD). This technique has been recently developed, and includes transmitting data via pressure pulses or mud pulses in the mud-circulation channel formed by the pipes during drilling. This is thus an acoustic transmission procedure. This procedure, like the procedure discussed above, has proved to be unreliable, very costly and very limiting. The procedure is limited to wells in which there is ongoing drilling. The procedure cannot be applied to production wells or wells in which tests are being performed, for example.
Electromagnetic transmission devices are applicable to all well configurations, such as drilling wells, wells being tested, wells in which production monitoring is required, etc. In the conventional mode of electromagnetic transmission, it has already been proposed to use a device which incorporates a transmission/reception aerial system that comprises an electrically insulating junction electrically separating the upper part of a drill string, which guides the signal, from the lower part of the drill string, the lower part acting as an antenna.
FIG. 1 represents a conventional MWD device for real-time measurement during drilling. A conventional dipole system is formed by an electrically insulating junction 1 which insulates an upper part 2 of a drill string from a lower part 3. The lower part 3 incorporates a terminal pipe equipped with a drill bit 4.
Inside of the drill string is a cylindrical element 5 enclosing, in a conventional manner, sensors, an electronic unit, and an energy source such as batteries. A modulated low frequency alternating electric signal is delivered between an upper pole P.sub.1 and a lower pole P.sub.2 located on the drill string incorporating the bit 4. The modulated signal, of several hertz, is representative of measurement performed by the sensors. The applied current has a value of some amps under a voltage of some volts.
Measurements which are presently made with this type of device include what is termed the geometric position of the tool in drilling operations when deviation from the vertical occurs, a wide-spread phenomena. The geometric position is delimited by three angles, that is, the inclination angle, the azimuth angle, and the tool face angle. Other useful measurements include the natural radioactivity of the soils (gamma ray), the pressure and temperature of the fluid inside and outside the drill string, and other measurements, such as the radioactivity caused by a neutron source, etc.
The signal applied between the poles P.sub.1 and P.sub.2 gives rise to the propagation of an electromagnetic wave in the soils surrounding the well. The electromagnetic wave is guided by the metal pipework formed by the upper string 2 and successive casings 6 and 7. Note that the "open hole" portion of the drill string is designated by reference numeral 12 below casing 7. The electromagnetic wave is guided by the metal pipework and is sent to the surface where it is collected by a transceiver 9. The transceiver 9 is connected first to the mass of a drilling apparatus 10, or to a well head, or to any other pipe in the well, and second to ground 11 positioned as far away as possible from the well, and which is generally at the bottom of the ocean in off-shore installations.
While this conventional system possesses the significant advantages of simplicity, reliability and utility, the possible distance of propagation of the electromagnetic waves depends closely on the average resistivity of the soils. The average resistivity, however, can vary considerably as a function of the drilling site. When the resistivity is approximately 10 ohms/meter, as is the case in secondary sedimentary ground in areas such as the Paris Basin or the center of the United States of America, a transmission over a distance of 3,000 to 4,000 meters may be achieved.
On the other hand, with the current state of the art, this technique can only be used in wells of a restricted depth (approximately 1200 to 1500 meters) for wells drilled in soils whose average resistivity approaches 1 ohm per meter, such as those found in pericontinental tertiary sedimentary soils, such as the soils found in the North Sea, the Gulf of Mexico and the Gulf of Guinea in West Africa. This transmission depth restriction is completely insufficient for most oil drilling operations, which extend to a depth of between 2000 and more than 4000 meters.